Alkaline divalent silver oxide cells have been known for a considerable period of time. However, a requirement for miniature, high energy capacity cells of the button cell variety--where there is a high power output and energy storage per unit volume--has recently arisen particularly in view of the wide acceptance of electrically powered watches and the like. There is a particular need for button cells having a high energy storage capacity and long shelf life, for such purposes as watches having a light emitting diode (LED) readout, LCD watches with backlights, electronic calculators, etc., where high energy pulses are demanded from the cell from time to time and where there is otherwise a low steady state current draw. In any event, either during its shelf life or its operating life, it is important that the cell demonstrate dimensional stability; and to do so, the rate of oxygen evolution within the cell must not be greater than the sum of the rates of oxygen diffusion to the anode of the cell of oxygen reaction with the anode, and oxygen escape from the cell.
Because of their high power output and energy storage per unit volume, divalent silver oxide cells are particularly attractive. However, divalent silver oxide is generally considered to be unstable in the presence of an aqueous electrolyte, such as any of the alkaline electrolytes generally used in commercial silver oxide cells. It has particularly been desirable to improve the stability of divalent silver oxide cells so as to take advantage of the characteristics stated above. One approach has been by Samuel Ruben in the U.S. Pat. No. 2,542,710 issued Feb. 20, 1951. In that patent, Ruben recognized that a steel or nickel substrate on the cathode can of a button cell tends to decrease the stability of the divalent silver oxide which is used in the cathode; and Ruben provided a silver plating or cladding on the cathode can to offset the tendency to decreased stability.
Dawson, in U.S. Pat. No. 3,484,294, issued Dec. 16, 1969, discloses a cell having a cathode which consists principally of divalent silver oxide over which a masking layer of monovalent silver oxide is placed, and above which a cellophane barrier is located between the monovalent silver oxide layer and an electrolyte absorbent layer. The Dawson cell, however, requires that the secondary active material layer--the monovalent silver oxide layer--be electrolyte impermeable; and it is difficult to produce such a cell in commercial quantities. In addition, the cell is subject to dimensional instability due to internal gassing--i.e., oxygen evolution--at greater rates than the recombinant and/or out-gassing rates.
Davies, in U.S. Pat. No. 3,853,262 issued Dec. 10, 1974, discloses a gold additive which is incorporated in the cathode or the cathode compartment of a divalent silver oxide cell. The gold additive is said to improve the stability of the cell, but has the disadvantage of increasing the cost of the cell.
Tvarusko, in U.S. Pat. No. 3,650,832, issued Mar. 21, 1972, proposes the addition of mercury, selenium and tellurium to a divalent silver oxide cell cathode, to improve the stability and/or the electrical conductivity thereof. The method of incorporating the additives is by physical admixture or by chemical coprecipitation, during the preparation of the divalent silver oxide.
Ruetschi et al, in U.S. Pat. No. 3,057,944, issued Oct. 9, 1962, teach a silver oxide cathode in a primary or secondary system where a surface active heteropolar substance is admixed either to the electrolyte or the silver cathode in an amount ranging from about 0.001 gram to about 0.2 gram per gram of silver; whereby there may be formed a polar chemical group which can attach itself to the surface of the silver oxide particles of the cathode, with the remainder of the molecule being hydrophobic in nature so as to hinder contact with the electrolyte. Excessive gassing of the divalent silver oxide when wetted by an alkaline electrolyte is reduced.
In all of the above prior art cells, as in the present invention, the anode or negative electrode is generally comprised of an amalgamated zinc powder; and the alkaline electrolyte is an aqueous solution of potassium hydroxide, sodium hydroxide, or a mixture thereof. Additional additives may also be included in the anode or electrolyte.
It has been noted that certain of the prior art divalent silver oxide cells have been particularly subject to dimensional instability due to excessive gassing of the divalent silver oxide cathode material. During storage or operation of such a cell, there is some spontaneous reduction of the divalent silver oxide to monovalent silver oxide, and at the same time there is a production of gaseous oxygen. A button cell can stay dimensionally stable, provided that the rate of oxygen evolution is not greater than the sum of the rates of oxygen diffusion to the anode of the cell or oxygen reaction with the anode, and oxygen escape or outgassing from the cell. Such increased internal gas pressure may cause physical separation of internal components within the cell, thereby permitting a particulate transfer of active material within the cell and thereby increasing the risk of an internal short circuit; or, in any event, there may be caused a bulging of the cell container to a greater extent than can be accommodated in its working position such as within an electroniccally operating watch case.
It has been found that the dimensional stability of divalent silver oxide cells, where the principal cathode material is divalent silver oxide or mixtures of divalent silver oxide with monovalent silver oxide, may be greatly enhanced, in keeping with the commercial requirements therefor and in accordance with this invention, by the reaction of silver oxide powder mixture which comprises at least 10% divalent silver oxide, the remainder including at least a minor amount of monovalent silver oxide, with at least a minor amount of a reagent which is a fatty acid, a salt of a fatty acid where the fatty acid has a chain length of at least twelve carbon atoms followed by removal of the fatty acid. In so doing, it has been found that a silver oxide powder mixture which has been reacted with a fatty acid or a fatty acid salt, has a much lower rate of oxygen evolution when wetted by an alkaline electrolyte, and a lower internal friction during powder flow under pressure than a silver oxide powder mixture which is known not to have had at least a minor amount of a fatty acid or a fatty acid salt reacted therewith. In addition, a cathode pellet which has been compressed from a silver oxide powder mixture treated as disclosed herein also displays a lower rate of oxygen evolution when wetted by an aqueous alkaline electrolyte.
Thus, the present invention provides not only a silver oxide powder mixture for use in the cathode of an alkaline divalent silver oxide galvanic cell, but also a method of preparing and producing the cathode pellet, and a cell which includes the cathode pellet; all so as to exhibit greatly improved dimensional stability characteristics with respect to prior art cells, or with respect to similar cells having cathodes prepared from similar silver oxide powder mixtures which are known not to have been reacted with at least a minor amount of a reagent which is a fatty acid or a fatty acid salt where the fatty acid has a chain length of at least twelve carbon atoms.